Hitherto, thermal-refined steels, which are steels required to undergo a quenching and tempering treatment (thermal refining, treatment) after hot working, e.g., hot forging, have been used for parts required to have strength and toughness, such as automotive parts and structural machine parts.
However, thermal-refined steels, although excellent in terms of strength and toughness, have a problem in that the cost of heat treatment for the quenching and tempering treatment (thermal refining treatment) performed for part production after hot working is high. In addition, there are problems in that since the martensitic transformation results in large heat-treatment strains, a large amount of machining is necessary for correcting the shape and dimensions after the heat treatment, resulting in an impaired yield, and that since this machining is given to the work which is in a hard martensitic state, the work shows poor machinability (workability), resulting in a prolonged period for part production and in an increased cost.
Microalloyed steels are steels which, in the as-hot-rolled state (specifically, in the state of having been hot-worked and subsequently cooled mainly by air cooling), exhibit required hardness and which can have the desired strength even when the quenching and tempering treatment after the hot working is omitted. Microalloyed steels are hence extensively used in applications such as structural machine parts, etc., as thermal-refined-steel substitute materials capable of accommodating cost reductions.
As such microalloyed steels, there is a ferrite/pearlite type microalloyed steel obtained by adding a slight amount of V to a medium-carbon steel. However, for enhancing the strength of the ferrite/pearlite type microalloyed steel to a certain level or above, it is necessary to heighten the areal proportion of pearlite to such a degree that the steel is substantially constituted of a pearlite phase alone.
In this case, however, since this steel has a structure consisting mainly of pearlite, which is brittle as compared with ferrite, the steel undesirably has considerably reduced toughness. It is hence difficult to enhance the strength to a certain level or above while ensuring toughness.
Among the microalloyed steels, there is a bainitic microalloyed steel which, in a hot-worked state, has a bainite structure. This steel has better toughness than the ferrite/pearlite microalloyed steel, but has a problem in that the proof stress thereof is low.
Merely heightening the hardness in order to improve the proof stress results in deterioration in machinability to increase the load in machining and impair the workability.
Age hardening type bainitic microalloyed steels are being investigated as one means for solution.
An age hardening type bainitic microalloyed steel is a steel which, in a hot-worked state, has a bainite structure and which comes to have heightened strength through a subsequent age hardening treatment. This age hardening type bainitic microalloyed steel, in the state of having undergone hot working and being soft, can be machined, and the hardness thereof can be heightened to required hardness by a subsequent age hardening treatment.
However, the conventional age hardening type bainitic microalloyed steels are still insufficient in toughness as compared with conventional thermal-refined steels, although the toughness thereof is better than that of ferrite/pearlite microalloyed steels.
Meanwhile, the investigations on conventional age hardening type bainitic microalloyed steels have aimed chiefly at increases in hardness and strength, and investigations for heightening toughness have not been made sufficiently.
Under such circumstances, Patent Document 1 describes an invention relating to “a steel part with high fatigue strength and high toughness for machine structure, and a process for producing the same” and discloses that an improvement in toughness was attained by refining bainite laths.
However, the improvement in toughness disclosed in Patent Document 1 is concerned with improvement in impact property (Charpy impact value), and the disclosed steel is still insufficient in fracture toughness, which is a toughness property different from impact properties, and is difficult to apply to parts required to have fracture toughness.
Furthermore, the steel described in Patent Document 1 requires a high cooling rate, and this imposes a large limitation on the production thereof.
With respect to other background-art techniques which are relevant to the present invention, Patent Document 2 describes an invention relating to “a steel for carburizing and carbonitriding” and discloses that, in a steel for carburizing and carbonitriding which is for use in gear and shaft applications required to have both high pitching fatigue strength and high impact strength, the temper hardness is improved by increasing the content of Si and the fracture toughness value of the carburized phase and core portion is improved by adding Ni or Mo alone or Ni and Mo in combination.
However, the steel described in Patent Document 2 basically differs from the steel of the present invention in that the former is not an age hardening type bainitic microalloyed steel.
Patent Document 3 describes an invention relating to “a rolled steel bar for hot forging and a material roughly shaped by hot forging” and discloses that, in case where the value of expression Fn1, which is a parameter serving as an index of the influence on tensile strength, exceeds 1.20, bainite is formed after hot forging in the material roughly shaped by the hot forging, resulting in a decrease in fracture toughness value, and alloying components are hence regulated so that the value of Fn1 is 1.20 or less.
However, the steel disclosed in Patent Document 3 differs from the steel of the present invention in that the former steel has a ferrite/pearlite structure and is not an age hardening type bainitic microalloyed steel and that the former steel has a content of Ni as low as 0.20% or less.
Patent Document 4 describes an invention relating to “an age hardening steel” and discloses an age hardening steel which, before an age hardening treatment, has a bainite-structure areal proportion of 50% or higher and in which the hardness increases, through the age hardening treatment, by 7 HRC or more from the hardness before the age hardening treatment.
The steel described in Patent Document 4 may contain Ni as an optional additive component, and the content thereof has been specified in the claims so as to be 1.0% or less. However, there is no Example in which Ni has been added. The steel described in Patent Document 4 substantially is a steel containing no Ni, and is different from the steel of the present invention.